Federal prosecutors investigating media-buying practices in the ad industry have begun issuing subpoenas as part of the probe, according to the Wall Street Journal.

One ad agency under scrutiny in the investigation is media conglomerate Vivendi-owned Havas, reports WSJ.

The investigation involves the FBI looking at the ad industry’s media-buying practices including accusations of agencies receiving rebates from media outlets.

Federal prosecutors investigating media-buying practices in the ad industry have begun issuing subpoenas as part of the probe, the Wall Street Journal reported Thursday, citing people familiar with the matter.

One ad agency under scrutiny in the investigation is media conglomerate Vivendi-owned Havas, according to the Journal. A spokeswoman for Omnicom told the Journal that the firm hasn’t received a subpoena from federal prosecutors.

The entire media buying industry has been under heavy scrutiny for several years